In antenna arrangements having both transmitting and receiving antennas, energy from the fundamental field formed by the transmitting antenna is often coupled into the receiving antenna. As can be appreciated, such coupling of energy reduces the effectiveness of the receiving antenna.
U.S. Pat. No. 4,135,183 describes an antenna arrangement which attempts to minimize this undesired coupling. More particularly, in the antenna arrangement of the '183 patent, a "figure-8" shaped transmitting antenna is provided wherein the top and bottom coils or loops are of opposite phase. The receiving antenna is also shaped as a "figure-8" with its axis perpendicular to the axis of the transmitting antenna.
Due to the opposite phasing of the loops of the transmitting antenna and the orientation of the receiving antenna with respect thereto, the effective coupling between the receiving antenna and the field produced by the transmitting antenna is substantially zero. Therefore, the pickup at the receiving antenna of the fundamental field produced by the transmitting antenna is minimized.
The above-described "figure-8" antenna arrangement, while minimizing coupling between the transmitting and receiving antennas, also produces unwanted nulls in both the transmitted field and the receiving pattern. A further undesirable effect of the "figure-8" antenna arrangement is that the energy coupled into the receiving antenna induces a current within the antenna which results in a field which opposes and diminishes the magnetic field produced by the transmitting antenna. This disrupts operation of the transmitting antenna and, further, is wasteful of energy.
In the '183 patent, the "figure-8" antenna arrangement is used in a harmonic-type magnetic EAS system. In such a system, electromagnetic energy is continuously transmitted at a fundamental frequency into a predetermined surveillance zone. In response to the transmitted electromagnetic energy, an EAS tag, when positioned within the surveillance zone, generates a signal at a harmonic of the fundamental frequency. This signal is detected by the receiving antenna and appropriate action is initiated, e.g., sounding an alarm.
However, due to the above-discussed nulls created by the "figure-8" configuration in the transmitted field, EAS tags positioned at these nulls do not generate signals and, hence, are not detected. Furthermore, as also above noted, a current is induced within the receiving antenna which induces fields which reduce the fundamental field and waste energy.
In another type of EAS system, known as a magnetomechanical type of EAS system, which is described in U.S. Pat. Nos. 4,510,489 and 4,510,490 and U.S. Pat. No. 5,051,726, the effects of coupling between the transmitting and receiving antennas of the system are minimized by a timing arrangement. In this system, the fundamental field generated by the transmitting antenna is pulsed on for a period of time and then is turned off. After waiting for the transmitted energy coupled into the receiving antenna to dissipate, the receiving antenna is enabled. Since the coupled energy has dissipated, the receiving antenna can now detect signals generated by tags without interference.
In the above system, the need to wait before turning on the system receiving antenna is undesirable. More particularly, the signal generated by a tag is at a maximum when the transmitted field is initially turned off and then decays at an exponential rate. Accordingly, due to the delay in enabling the receiving antenna, the receiving antenna must be capable of detecting tag signals which are at a reduced level. This increases the sensitivity demands placed on the receiving system and/or limits the extent of the surveillance zone.
Furthermore, in magnetomechanical type EAS systems, for example, it is known to use two spaced transmitting antennas arranged in parallel planes defined by the vertical and lateral directions to radiate two different fundamental fields (fields due to two different antennas) along the antenna axes (i.e., in the horizontal direction) into the surveillance zone. These fields, moreover, are radiated such that they are 180.degree. out-of-phase. As a result, the fields subtract along the horizontal direction and add along the vertical and lateral directions. Accordingly, with the fields so generated, tags orientated vertically and laterally in the surveillance zone, result in additive voltages in serially connected, oppositely phased receiving antennas. Detection of vertically and laterally oriented tags is thereby enhanced.
It is also known in such systems to use the transmitting antennas to radiate two different fields from two different antennas such that the fields are in-phase. In this case, tags orientated horizontally in the surveillance zone result in additive voltages in in-phase, serially connected receiving antennas, thereby enhancing the detection of the horizontally oriented tags. Thus, by switching the phasing operation of the two transmitting antennas and the two receiving antennas, detection of tags oriented in the vertical, lateral and horizontal directions is facilitated.
Due to the delay required before enabling the receiving antennas in the aforesaid systems, the phasing of the two transmitting antennas cannot be rapidly changed. This slows down the overall system operation.
It is therefore an object of the present invention to provide an improved antenna arrangement in which the coupling between the transmitting and receiving antennas of the arrangement is reduced in a way which avoids the above problems of prior systems.
It is a further object of the present invention to provide an EAS system which uses the above improved antenna arrangement.
It is a still further object of the present invention to provide an EAS system using the above described improved antenna arrangement in which the phase of the transmitted fields can be rapidly switched.